A significant problem for advanced industrial, aircraft and marine gas turbines is the surface stability of superalloy components. Highly corrosive environments are generated by the combustion of heavy fuel oils, and when this is combined with higher firing temperatures and longer maintenance intervals, very strict limitations arise in material selection. It has become increasingly difficult to generate both creep-rupture strength and good corrosion resistance through alloy modification of the base metal alone. Thus, various coating and cladding schemes have been developed to provide surface protection to the superalloy substrate. The bonding of an oxidation and hot corrosion resistance sheet cladding to a turbine bucket or nozzle represents a solution to the surface stability problem.
Recently, considerable progress has been made in the development of methods for the diffusion bonding of cladding to a convex-concave substrate such as an airfoil or turbine blade. For example, Schilling et al, U.S. Pat. No. 3,928,901 teaches a method in which the sheet cladding is cold isostatically pressed to form a tight skin over the substrate. In Beltran, et al, U.S. Pat. No. 3,904,101, a process of cladding is disclosed in which the space between the cladding and the substrate is evacuated, all seams are vacuum brazed and thereafter the assembly is diffusion bonded in an autoclave using a gaseous medium and elevated temperature and pressure. Schilling et al, U.S. Pat. No. 3,952,939, teaches a process in which a preassembled sheet cladding and substrate are masked at all seams, surrounded with glass chips and then hot diffusion bonding while melting the glass and ensuring an isostatic stress state. These processes, heretofore employed, for providing claddings on convex-concave substrates involve only a single cladding layer.
An alternative method of protecting superalloy articles from high temperature oxidation and corrosion is by using coating techniques. Various coatings for superalloys have been described in the literature and of particular interest are coating compositions consisting essentially of chromium, aluminum, a member selected from the group consisting of yttrium and the rare earth elements, and a metal selected from the group consisting of iron, cobalt and nickel. These coating have been designated in the art as MCrAlY coatings.
Further coating improvements have been achieved by employing multiple coating layers. Thus Walker et al, U.S. Pat. No. 3,873,347 disclose coating a superalloy body with an MCrAlY coating by physical vapor deposition and then forming an aluminized overlayer by pack cementation. Rairden, U.S. Pat. No. 3,874,901, teaches a similar process with the exception that the aluminum overlayer is formed by physical vapor deposition. U.S. Pat. No. 3,649,225 teaches the use of a composite coating, in which a chromium or chromium rich interlayer is adjacent the superalloy substrate and over which is an outer MCrAlY layer. U.S. Pat. No. 4,005,989 discloses coating a superalloy substrate with an aluminide interlayer and then coating with an MCrAlY overcoat.
It is therefore an object of the present invention to provide a novel method of diffusion bonding double layers of sheet cladding to complex shaped superalloy components by a single bonding step and thereby eliminate a plurality of bonding steps such as those employed in the prior coating art.